The crown density results, using both methods ofassessment, are presented in 10% classes in Table 1.The marked effect of using a local reference tree ratherthan an ideal tree as the basis for comparison can beseen for all species. Much of the difference can beaccounted for by variations in growth habit betweenthe reference photographs of ideal trees (Innes, 1990)and the trees in and around the plots to be assessed,from among which a local reference tree is chosen. Forexample, young trees of all species, but particularlyScots pine, tend to have a more open appearance (i.e.a lower crown density) than the older trees illustratedin Innes (1990), and some older oaks and spruces alsohave a naturally open structure. For trees like this, theapparent reduction in crown density would thereforebe much greater when judged against an ideal tree thanwhen compared to local trees of the same age and form.
Figure 1 shows the changes in crown condition thathave taken place since 1987. An upward gradient inthis figure indicates a deterioration in crowncondition. In contrast to the method of presentationused before 1998, the figure records the mean percentreduction in crown density for each species comparedto an ideal tree. Changes in crown density comparedto last year were minor for all species except beech,the condition of which deteriorated significantly.Analysis of the time series for each species shows that,in general, only relatively large changes in crowndensity (more than 3% to 4%) between years arestatistically significant. Short-term changes of suchmagnitude, both positive and negative, havecharacterised beech over the fourteen year surveyperiod. However, there is no evidence of a long-termdeterioration or improvement of crown condition inthis species. Similarly, no long-term trends can bedetected in the crown densities of either Scots pine orSitka spruce, although the latter is the only speciescurrently in better condition than at the commencementof the survey in 1987. Analysis of the 1987 to 2000 dataindicates that a statistically significant deterioration incrown condition has occurred for both oak andNorway spruce over the duration of the survey.However, the time series is of relatively short durationand the indicated rates of change are small, with anapparent reduction in crown density of 0.57% perannum in oak and 0.47% per annum in Norwayspruce. The magnitudes of past increases in crowndensity in these species suggest that a single year ofimprovement could nullify the current trends.
Since 1991 the condition scores of Scots pine andNorway spruce have changed less than those of any ofthe other species. Although Norway spruce displayedno significant change in 2000, the condition of Scotspine showed a slight improvement. Following a sharpdecline in 1997, the condition of Sitka spruce began toimprove in 1999 and this recovery continued in 2000.Oak also improved this year, reversing the slightdeterioration recorded in 1999, and was in bettercondition than at any time since 1995. However, it isstill markedly poorer than during the late 1980s. Thedecline in beech recorded in 2000 completely reversedthe improvement recorded in 1998 and crown densitywas reduced to the lowest level recorded since 1991.
Table 1: Percentages of trees in each crown density class for five species in 2000. Each 10% class represents a reduction in crown density compared either to an 'ideal tree' (I), i.e. a tree with the maximum possible amount of foliage, or to a 'local tree' (L), i.e. a tree with full foliage under local conditions.
Figure 1: Changes in crown density since 1987 for five species surveyed annually. The crown density compared with that of an 'ideal' tree with a completely opaque crown is shown for each species.
Figure 2 shows the geographical variation in crowndensity for the five species assessed. Variation was greatest in oak, which has shown substantially the samepattern since 1997 when data were first presented inthis way (Redfern et al., 1998; 1999; 2000). Oak wasagain poorest in central Scotland and north-eastEngland, south-west England, Wales and East Anglia,and best in southern England. Scots pine also displayeda pattern that was similar to those in previous years,with crown density tending to be highest south of theHumber-Mersey line. Beech showed no clear pattern.Both spruces were poorest in the south and east thanelsewhere, but this impression is created by relatively fewplots and both species show considerable local variation.
Factors affecting crown condition in 2000
Over much of the country April and the beginning ofMay were extremely wet but the consistent cloud coverover this period contributed to the absence of damagingspring frosts. Unprecedented high winds in mid-Juneresulted in shoot loss and browning of current yearneedles in two Norway spruce and four Sitka spruceplots in the north of the UK. In spite of extremely dryweather over much of Scotland in July, no droughtdamage to forest trees was reported. The remainder ofthe growing season was generally warm and wet.
The deterioration in beech this year was of similarmagnitude to former declines in 1990 and 1995 (Figure1). As in these previous cases (Redfern et al., 1996), thereduction in crown density in 2000 was associated withheavy mast production (Table 2). Although thedeterioration in the condition of beech and theincreased fruiting noted in 1990 were considered asseparate manifestations of stress caused by the droughtof 1989 (Innes & Boswell, 1991), neither the 1995 nor2000 growing season were preceded by drought years.However, since female flowers and subsequently fruitsoccupy positions which might otherwise give rise tovegetative shoots bearing foliage, heavy fruiting alonemay bring about a reduction in crown density.
Although the condition of oak is better than forseveral years, it is still poorer than any of the otherspecies surveyed. As in previous years, this appears tobe due largely to insect damage which was recorded in74 of the 85 plots assessed. Plots with low crowndensity were generally those in which insect activitywas scored as heavy or severe, although oak dieback(Gibbs, 1999) appeared to be the primary cause ofpoor condition in four plots in England.
The condition of Sitka spruce improved slightlycompared to last year. There were few reports ofmajor damage by green spruce aphid and scores forinsect damage remained at the low levels recorded in1999, suggesting that Sitka spruce is continuing agradual recovery from the severe Elatobium outbreakof 1997. Exposure injury and abrasion by wind werereported from 13 Sitka spruce and 23 Norway spruceplots. The condition of Norway spruce was virtuallyunchanged in 2000, with only minor damage frominsects and fungi being recorded.
The improvement in Scots pine this year reflected a marked increase in the number of trees in the lowest defoliation classes (0–20% reduction in crown density) despite levels of damage from the pine shoot beetle(Tomicus piniperda) and by the fungi Lophodermiumseditiosum and Peridermium pini which were similar to those recorded in 1999. However, counts of needle retention demonstrated a marked increase in the number of trees retaining needles for 2 or more years in 2000.
Apart from a relatively short dry period over the northof the country in July, rainfall was well distributedthroughout the growing season and tree growth wasgenerally good. Unseasonal high winds in June causedlocalised damage to Sitka spruce and Norway sprucein Scotland but no other forms of climatic injury wereimportant this year. Changes in crown density wereminor in all species except beech, the condition of whichdeteriorated significantly. However, this change wasassociated with heavy mast production, which has beenrecorded during previous episodes of sharp reduction inbeech crown density, and is not necessarily an indicationof ill health. A slight improvement in condition thisyear suggests that Sitka spruce is continuing a gradualrecovery from the severe outbreak of Elatobium whichaffected it in 1997. Oak, although better in 2000 thanfor several years, is still in poorer condition than theother surveyed species. Crown density in Norwayspruce has displayed only minor fluctuations since1991 and remained virtually unchanged in 2000.Scots pine improved slightly, largely due to an increasein the number of needle years retained by trees.